The only reason one would use scope.$watch in this fashion is to have a function executed when a digest occurs. The original purpose of the $watch function is t

The only reason one would use scope.$watch in this fashion is to have a function executed when a digest occurs. The original purpose of the $watch function is to take 2 functions, one that returns the watched value and another to be executed when that value changes. The fiddle provided is not a very good example of "reusable" directives. The fiddle could be rewritten to use a directive that adds or removes a given class based on a boolean, and the rest of the state could be handled by built in directives.